Yes. You should definitely either switch or keep the Fascinate. Personally, I'd go with whichever phone I liked better
It really is a personal choice. What don't you like about the Fascinate? What do you like about the D2? For me the only thing I like about the D2G is that it's got a GSM radio, if the DX had one I'd probably go there for both phones (or, more likely, someplace else for my play phone) because of the larger screen and lack of keyboard. Many (most?) people here love the D2 keyboard, I'm sure it's also why Pro owners went with their phones but the keyboards are very different. Or are you looking for CPU horsepower? If so, what applications do you use to stress the CPU (graphics vs. calculations)?
Will the D2 make you happy? Should you wait for the next generation phones? Are you moving to Motorola or away from Samsung? Do you like the interface on the Motorola better than the one on HTC or LG? All of these are questions only you can answer. Here you'll hear that the D2 and D2G are great phones with excellent build quality. Of course you'll probably hear a lot of similar things on the Samsung forums.